1. Field of the Invention
The present invention relates to a charge transfer device for transferring a signal charge and converting and outputting a signal charge in the form of an electrical signal. More particularly, this invention relates to a charge transfer device for use as an area sensor, a linear sensor or a delay element.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows an arrangement of an example of an area sensor which is one of CCD (charge coupled device) solid state imaging devices using a charge transfer device.
As shown in FIG. 1, an area sensor comprises a plurality of photoelectric conversion elements (referred to hereinafter as "photosensors") arrayed in a matrix fashion, a plurality of vertical transfer registers 12 arrayed at every vertical column of the photosensors 11 for transferring signal charges read out from the photosensors 11 in the vertical direction, a horizontal transfer register 13 for transferring the signal charges transferred from these vertical transfer registers 12 in the horizontal direction and a charge detecting portion or charge converting means 14 for detecting the signal charge transferred by the horizontal transfer register 13 and converting and outputting the same in the form of a signal voltage.
The charge detecting portion 14 is formed of a floating diffusion (FD) region and reset in response to a reset pulse .phi.rs supplied thereto at every predetermined period. An output of the charge detecting portion 14 is developed through an output buffer 15 as an output voltage Vout.
FIG. 2 is a diagram of waveforms of the reset pulse .phi.rs and the signal output Vout.
In this kind of the charge transfer device, a relationship between an amount (intensity) of light incident on the photosensor 11 (or charge amount of resultant signal charges) and the corresponding output voltage Vout of the output buffer 15, i.e., amount of light (charge amount)--output voltage characteristic is considerably fluctuated with states, such as power supply voltage and temperature.
FIGS. 3A, 3B are graphs showing measured results of amount of light (charge amount)--output voltage characteristics. FIG. 3A shows the amount of light (charge amount)--output voltage characteristic obtained in the standard state. FIG. 3B shows the light amount (charge amount)--output voltage characteristic obtained when the state (e.g., temperature) was changed.
Study of the characteristic diagram shown in FIG. 3B reveals that the output voltage Vout tends to increase as the temperature rises. This tendency becomes remarkable particularly when an amount of light is small. However, although the temperature is low, the output voltage Vout increases in the portion with large intensity and an inclination of a straight line changes minutely in response to the temperature. Also, when the power source voltage is fluctuated, it is natural that the charge amount--output voltage characteristic also is fluctuated.
As described above, it is to be appreciated that, when the amount of light (charge amount)--output voltage is considerably fluctuated with the power source voltage and the temperature, the output voltage Vout corresponding to the same amount of light (charge amount) also is considerably fluctuated with a fluctuation of the power source voltage and the change of temperature. Therefore, when the above-mentioned characteristic is fluctuated, it is necessary to correct the output voltage Vout.
There has not hitherto been proposed a proper method of detecting a fluctuated amount of the characteristic and correcting the fluctuated characteristic when the amount of light (charge amount)--output voltage characteristic is fluctuated.
Therefore, an object signal serving as a white reference is read out each time or one time in a certain state (power source voltage and temperature), and an amount of light (charge amount)--output voltage characteristic is corrected by comparing the signal level and the signal level of the black level (black level of the output signal, signal level of an optical black (OPB: optical black) or signal level of an object which becomes black reference).
According to such a method, however, a signal charge correspond to the object which becomes white reference should constantly be input or input each time the state is changed, otherwise the characteristic cannot be detected. Also, in actual practice, it is difficult to stably pick up put light from the object which becomes white reference during one field period because brightness of the object does not always become proper for white reference when the cameraman starts taking a picture in practice.